The two longtime friends are reported to be “smitten.” (Dave Allocca/Startraksphoto.com)

Michelle Williams and Jason Segel are a couple, reports say.

A source tells Us Weekly that the longtime friends have fallen hard for each other.

“They are smitten and very serious,” the source says. “[Williams] hasn’t been this happy in a long time.”

The source’s claim is backed up by new images of the two, which show Williams and Segel strolling hand-in-hand through Brooklyn’s Carroll Gardens on Sunday, where they dined at Frankie’s 457 Spuntino restaurant.

“He put his arm around her and made her laugh as they walked,” a source tells the magazine. “[They] seemed very relaxed.”

The next day, Segel was spotted out on the street with Williams’ daughter, Matilda Ledger, watching the 6-year-old ride her scooter in a pink jacket and helmet.

The “My Week with Marilyn” star was in a long-term relationship with Heath Ledger, who she met while filming “Brokeback Mountain,” only splitting months before his untimely death in 2008. Williams later dated director Spike Jonze for about a year.

Meanwhile, “How I Met Your Mother” star Segel has been briefly linked to both Michelle Trachtenberg and Chloe Sevigny, and was most recently in a long-term relationship with actress Michelle Ryan.

While Williams and Segel have been friends for years, they reportedly met through actress Busy Phillips, who Williams brought as her date on the awards show circuit this year while being honored for “My Week with Marilyn.” Phillips co-starred with Segel in the cult-classic TV show, “Freaks and Geeks.”